A conventional arrangement is described in German Patent Application No. 38 12 633. In this conventional arrangement, a rotary transformer is used, for example, for signal transmission between a controller and an air bag built into the steering wheel of a motor vehicle. The operability of an air bag must be constantly monitored, so that it is ready to be used immediately in an emergency situation. For this purpose, the resistance of the squib, which should be typically between 1.8 ohm to 2.5 ohm, must be periodically measured. Signal transmission between the inductively coupled windings of the rotary transformer depends on the spacing between the two windings, i.e., the size of the air gap between the pot-type cores of the windings. Installation and manufacturing tolerances of the steering wheel, on which the secondary pot-type core of the rotary transformer is installed, can result in distortions in the measured resistance value of the squib, as can the operating temperature. In order to avoid such distortions, the above-described conventional arrangement provides an oscillator circuit on the secondary side of the rotary transformer, which is excited by a control signal from the primary side. After the control signal is switched off, the decay of the response signal of the oscillator circuit is transmitted back to the primary side and the secondary-side resistance is determined from the time constant of the decaying response signal.
The object of the present invention is to provide a device allowing reliable information to be derived from the primary-side input voltage of the transformer concerning the status (resistance) of the load connected to the secondary side.